The purpose of this project is the quantitative understanding of the molecular control governing the differentiation of muscle cells. The messenger RNA transcribed from the genes responsible for the differentiation of muscle cells is the first indication when specific genes are activated. The regulation of this messenger RNA results in the regulation of the time of synthesis of the proteins responsible for, and characteristic of, muscle differentiation. Messenger RNA for these muscle proteins is stable whereas most messenger RNA species within the cell are not stable. An understanding of the mechanisms controlling messenger RNA stability, and the kinetics of synthesis and decay of particular messenger RNA species is vital to the understanding of differentiation, and in fact, any cellular process under programmed genetic control. Loss of this control may be a mechanistic definition of many cellular disease processes.